At its basic roots, the subject matter of rotary wing structural dynamics and aeroelasticity is concerned with the following problem: How can structural integrity and passenger comfort be attained in the vibratory environment peculiar to rotary wing aircraft? From this broad problem definition has evolved the two principal areas of concern in rotary wing structural dynamics: vibrations and aeroelastic stability. The emphasis is currently on vibration reduction, but with close attention being paid, nonetheless, to the fact that rotors are subject to a variety of potentially unstable response phenomena. The advent of new rotor concepts increases the probability of an otherwise benign characteristic rotor response becoming a major aeroelastic instability issue. In any practical design the instabilities must be well understood, and ways must be found to suppress them in all conceivable flight conditions. Last, it should be noted that one area of concern that closely relates to structural dynamics and aeroelasticity is that of noise. The high degree of noise characteristic of most rotorcraft has spawned the growing technical areas of rotor far-field noise acoustics and structure-borne interior noise acoustoelasticity. Although both these subjects are important and timely, this text is limited to considerations of only vibrations and aeromechanical and aeroelastic instabilities.